Halogenated sulfidohydroboranes for nuclear medicine and boron neutron capture therapy

ABSTRACT

A method for performing boron neutron capture therapy for the treatment of tumors is disclosed. The method includes administering to a patient an iodinated sulfidohydroborane, a boron-10-containing compound. The site of the tumor is localized by visualizing the increased concentration of the iodine labelled compound at the tumor. The targeted tumor is then irradiated with a beam of neutrons having an energy distribution effective for neutron capture. Destruction of the tumor occurs due to high LET particle irradiation of the tissue secondary to the incident neutrons being captured by the boron-10 nuclei. Iodinated sulfidohydroboranes are disclosed which are especially suitable for the method of the invention. In a preferred embodiment, a compound having the formula Na 4  B 12  I 11  SSB 12  I 11 , or another pharmaceutically acceptable salt of the compound, may be administered to a cancer patient for boron neutron capture therapy.

This invention was made with U.S. Government support under Department ofEnergy Contract Number DE AC02-76CH00016 and under National Institutesof Health Grant Number IR01CA38107-01A2. The U.S. Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to boron-containing compounds useful inboron neutron capture therapy of tumors and to methods for using suchcompounds. The compounds of the present invention provide a means fordirectly quantifying boron concentrations in tissue, thereby permittingrapid enhanced targeting and planning of neutron-capture irradiation oftumors.

2. Background of the Related Art

Cancer continues to be one of the foremost health problems. Conventionaltreatments such as surgery, photon radiation therapy, and chemotherapyhave been successful in certain cases and unsuccessful in others. Anunfamiliar form of radiation therapy for cancer, known as BoronNeutron-Capture Therapy (BNCT), is being investigated to treat certaintumors for which the conventional methods have been ineffective. Forexample, BNCT has been used clinically in Japan to treat glioblastomamultiforms, a highly malignant, invasive form of brain cancer.

BNCT is an anti-cancer bimodal radiation therapy that utilizes theability of the stable (non-radioactive) nucleus boron-10 (¹⁰ B) toabsorb thermal neutrons. In BNCT of malignant brain tumors, the patientis first given an infusion of a boron-containing compound that is highlyenriched in the ¹⁰ B isotope. Ideally, the boronated compoundconcentrates preferentially in the brain tumor. For some boron compoundsunder investigation in BNCT research, the action of theblood-brain-barrier generally minimizes their entry into the surroundingbrain tissues. The tumor area is then irradiated with a beam of thermalneutrons (primary irradiation), some of which are captured by theboron-10 concentrated in the tumor. The relative probability that theslow-moving thermal neutrons will be absorbed by the boron-10 nuclide ishigh compared to the probability of absorption by most all othernuclides in the tissue combined, i.e., the nuclides normally present inmammalian tissues. Boron-10 undergoes the following nuclear reactionwhen captured by a thermal neutron:

    .sup.10 B+n→*.sup.11 B,

    *.sup.11 B→.sup.7 Li+.sup.4 He+γ(478 keV)

In this nuclear reaction, a ¹⁰ B nucleus absorbs a neutron, forming themetastable nuclide *¹¹ B, which spontaneously disintegrates into a ⁴ Heparticle and a ⁷ Li particle, bearing a total kinetic energy of 2.34MeV. These two particles have 9 μm and 5 μm ranges in tissue,respectively. Accordingly, the particles are capable of destroying cellssuch as cancer cells, and/or cells of the blood vessels in the tumorthat allow the cancer to grow, the nuclei of which are in theirtrajectories. In effect, the tumor alone is preferentially irradiatedwith these high linear energy transfer (LET) alpha and ⁷ Li particleswhose range in tissue is about 10 μm, a distance comparable to thediameter of an average cell. Therefore, the efficacy of BNCT resides inthe production of highly localized, ionizing radiation in the targettissue. In this manner, the tumor receives a relatively large radiationdose, compared to that received by the surrounding healthy tissue.Optimally, the preferential accumulation of boron-10 in the tumorpermits the radiation dose to the tumor to exceed the dose to the bloodvessels of the surrounding normal brain and to greatly exceed the doseto the extravascular normal brain tissue.

Several criteria must be met in order for radiation enhancement providedby BNCT to be successful. First, the ¹⁰ B must be present in significantquantities at the tumor site (at least about 10 mg, and preferably morethan about 30 mg ¹⁰ B/g tissue). Second, there should be highselectivity of the drug for the tumor over normal tissue, withtumor-to-normal tissue ratios greater than two. Generally, this secondcriterion is satisfied if the boronated drug does not penetrate theblood brain barrier. Third, the tumor-to-blood ratio should be not lessthan one. Fourth, the boronated drug should not be significantly toxicto the patients being treated. However, considering the seriousness ofmalignant brain tumors, moderately toxic chemotherapeutic agents andother therapeutic agents are widely used.

BNCT differs from other cancer therapy modalities. For example, BNCTdiffers from conventional radiotherapy modalities because it uses anexternal beam to produce a high radiation dose only where a chemicalcompound has accumulated prior to irradiation. BNCT also differs fromchemotherapy because the compound that accumulates in the tumorexpresses its tumoricidal action only within the field of the neutronradiation beam. The efficacy of BNCT therefore depends not only upon therelative concentrations of boron in the blood, in the tumor and in othervital tissues within the treatment volume, but also depends upon thequality of the neutron beam.

The concentrations of ¹⁰ B within the tissues of patients have beenestimated indirectly by pharmacokinetic extrapolation from theconcentration of ¹⁰ B in blood and tissue samples from patients and fromexperimental animals. Such extrapolations are approximate. It wouldtherefore be desirable to have a method for more directly, more rapidly,and more accurately determining the concentration and distribution of ¹⁰B in a patient being prepared for BNCT. The implementation of rapid BNCTtreatment planning, enabled by relatively common and cost effectiveimaging techniques such as computerized tomography (CT) and nuclearmedicine scintigraphy, would greatly facilitate the clinical acceptanceand efficacy of BNCT.

The simultaneous labeling of antibodies with ¹⁰ B and with othernuclides, including iodine, for purposes of imaging tumors and targetingthermal neutron radiation exposures are described in U.S. Pat. Nos.4,348,376 to Goldenberg, 4,665,897 to Lemelson, and 4,824,659 toHawthorne. Each of these patents, however, requires that the ¹⁰ Bcompound be linked to a radio-labeled antibody. None of these patentsdiscloses boron compounds that are targeted nonspecifically, nor do theydisclose nonradioactive iodination of boron-containing compounds forimaging. In addition, the iodine is described in these patents as beingdirectly substituted for hydrogen atoms on the antibody molecule, butnot into the borane moiety.

Specifically, U.S. Pat. No. 4,348,376 to Goldenberg describes methodsfor radiolabeling antibodies to carcinoembryonic antigen (CEA), a cellsurface marker commonly associated with certain types of tumors. Theanti-CEA antibodies are described as being coupled to a ¹⁰ B-containingmoiety including, for example, the diazonium ion derived from1-(4-aminophenyl)-1,2-dicarbacloso-dodecaborane. Goldenberg furtherdescribes methods for radiolabeling the boron-rich antibody complexusing any of a group of radioisotopes that emit detectable particle orphoton radiation, for example, iodine-131 (¹³¹ I), iodine-123 (¹²³ I),or iodine-125 (¹²⁵ I).

U.S. Pat. No. 4,665,897 to Lemelson discloses a method of radiolabelingantibodies, desirably antibodies specific for tumors. Lemelson describesa variety of boron-containing moieties that may be coupled to theantibodies. Lemelson also describes the additional coupling ofradionuclides, including isotopes of iodine, to the antibody moiety ofthe boron-coupled antibodies. The isotopes used for imaging of thetarget tissues are described by Lemelson as either stable orradioactive, allowing for targeting by imaging of either stimulated orspontaneous emission of radiation. Lemelson does not describe boranes asadded moieties or the iodination of boranes, nor does Lemelson describea method for adding sufficient stable iodine to antibodies in amountsthat would allow chemical noninvasive imaging while also allowing theantibodies to retain their antigenic specificity for a tumor.

U.S. Pat. No. 4,824,659 to Hawthorne describes the modification ofantibodies by coupling them to a synthetic poly(amide/urea/thiourea)moiety containing any of various boranes, resulting in antibodyconjugates carrying 50-2000 boron atoms with about 96% ¹⁰ B content. TheHawthorne patent describes a variety of other antibody conjugates,including antineoplastic agents, paramagnetic spin labels, chromogens,etc., in addition to ¹⁰ B borane compounds. Hawthorne also describes thecoupling of radionuclides, including isotopes of iodine, to the antibodymoiety of the borane-antibody complex, for in vivo diagnostic use.

The use of radioisotope-labeled therapeutic substances for purposes ofPET imaging of blood concentration and feedback control of the rate ofadministration of the labeled substances is described in U.S. Patent No.4,409,966 to Lambrecht et al. The Lambrecht et al. patent, however, doesnot disclose compounds containing boron for BNCT. Lambrecht et al. alsodescribe the application of their method to the injection of otherradiopharmaceuticals. Unlike the present invention, Lambrecht et al.employ a pharmaceutical labeled with a positron-emitting isotope fordetection by Positron Emission Tomograph (PET). Lambrecht et al. do notdescribe the use of ¹⁰ B carriers as an aspect of their method. Nor dothey describe the use of the technique for imaging drug concentrationsaround tumors or for the targeting of radiation therapy.

Thiouracil derivatives of decaboranes used for BNCT are described inU.S. Pat. Nos. 5,116,980 and 5,144,026 to Gabel. These patents disclosea variety of compounds and their intermediates that may be used forBNCT, but do not describe labeling of the compounds for imagingpurposes. They disclose halogenated boranes such as omega-carboranylacyl halides. Gabel does not describe the use of these compounds astherapeutic agents. Rather, the compounds are described as intermediatesin the reactions required to produce the therapeutic thiouracilderivatives. Further, Gabel does not disclose imaging of bloodconcentrations of ¹⁰ B carriers. The Gabel patents do not describe theuse of nuclide-labeled ¹⁰ B carriers for simultaneous imaging andtherapeutic use in BNCT.

A review of the history of BNCT as applied to brain tumors is providedin a publication by one of the co-inventors, Slatkin, "A History ofBoron Neutron Capture Therapy of Brain Tumours", Brain 114 1609-1629(1991). This article describes uses of ¹⁰ B carriers before 1991, suchas sulfidohydroboranes, including Na₂ B₁₂ H₁₁ SH (a monomer hereinafterabbreviated BSH) and Na₄ B₂₄ H₂₂ S₂ (a dimer hereinafter abbreviatedBSSB). Slatkin mentions the need for techniques capable of generatingdata on the distribution of BNCT agents in patients, but does notdescribe any means for noninvasive imaging of ¹⁰ B-carriers. Slatkinalso mentions the use of BSH and BSSB in clinical studies in Japan. Seealso Joel et al,. "Boron Neutron Capture Therapy of Intracerebral RatGliosarcomas", Proc Natl. Acad. Sci. USA, 87, 9808 (1990). See also U.S.Statutory Invention Registration No. H505 to Slatkin et al.

Chemical methods for halogenating, for example iodinating, decaboranesand dodecaboranes are described by Knoth et al., "Chemistry of Boranes.IX. Halogenation of B₁₀ H₁₀ ⁻² and B₁₂ H₁₂ ⁻² ", Inorg. Chem., 3(2),159-167 (1964). The article is limited to the particular syntheticmethods described, and does not describe or suggest any application ofiodinated boranes to BNCT. Unlike the present invention which describesthe use of sulfidohydroboranes, Knoth et al., do not describe or suggestsulfur-containing boranes in that particular article, although Knoth isthe principal author of the first publication that described thesynthesis of BSH, see Knoth et al., "Chemistry of Boranes. XIX.Derivative Chemistry of B₁₀ H⁻² and B₁₂ H₁₂ ⁻² ", J. Am. Chem. Soc., 86,3973-3983 (1964).

Therefore, it would be advantageous to improve prospects for clinicalBNCT that facilitates the selective uptake by tumors of a non-specifichalogenated ¹⁰ B-carrier and the simultaneous visualization ofiodine-labeled ¹⁰ B concentrations within a patient for purposes oftargeting a therapeutic neutron beam.

Accordingly, it is a purpose of the present invention to provideiodinated, boron-containing compounds that are useful in BNCT and thatsimultaneously enable visualization of the compound for purposes ofrapidly and directly targeting the tumor and estimating the neutronirradiation dose.

It is also a goal of the present invention to provide an improved methodof BNCT in which an iodinated boron compound serves as the therapeuticagent and, at the same time, enables the targeting of a neutron beam.

It is a further purpose of the present invention to provide methods forthe synthesis of iodinated sulfidohydroborane compounds useful for BNCTand visualization of the boron biodistribution of these compounds invivo.

SUMMARY OF THE INVENTION

These and other purposes and goals are achieved by the present inventionwhich solves the disadvantages inherent in the prior art by providing aclass of ¹⁰ B-containing compounds useful for boron neutron capturetherapy (BNCT) and a method of using ¹⁰ B-containing compounds for BNCT.The class of compounds according to the present invention includesiodinated closo sulfidohydroborane monomers and dimers. In oneembodiment, the monomeric compounds of the invention have the formulaB_(m) I_(n) H_(m-n-1) SH, in which m is an integer between 5 and 12 andn is a positive integer less than m. In another embodiment, the dimersof these compounds have the formula B_(x) I_(y) H_(x-y-2) S₂, in which xis an integer between 10 and 24 and y is a positive integer less than orequal to x-2. Two of the preferred compounds of the invention are aperiodinated monomeric sulfidohydrododecaborane (IBSH) (m=12, n=11) anda periodinated dimeric sulfidohydrododecaborane (IBSSB) (x=24, y=22).The dimeric compounds of the invention include two monomeric boraneslinked through a disulfide bridge formed through the oxidation ofsulfhydryl groups present on the monomers.

The compounds of the invention should contain at least a naturalabundance of boron-10 isotope. Preferably, at least 95% of the boronatoms in the compounds are boron-10 atoms.

The iodine isotopes useful for the invention may be radioactive,preferably iodine-123 (¹²³ I), but can also include ¹³¹ I, or ¹²⁵ I.Alternatively, the iodine isotopes may be radiostable, preferablyiodine-127 (¹²⁷ I). The compounds of the invention may includeradiostable and radioactive nuclides of iodine on a single boranemolecule. The incorporated iodine isotopes permit imaging of theiodinated sulfidohydroboranes by an imaging technique such as computedtomography (CT) or single photon emission computed tomography (SPECT).The present invention also includes methods for preparing iodinatedsulfidohydroboranes as therapeutic agents for use in BNCT.

The invention also includes a method of performing BNCT, which includesadministering a boron compound of the invention to a patient; locatingthe patient's tumor by scanning the patient with a device capable ofdetecting the location and extent of boron uptake by the tumor; anddirecting a beam of thermal neutrons toward the tumor to permit neutroncapture by the boron and the subsequent secondary irradiation of thetumor with the fission products of metastable ¹¹ B decay, i.e., alphaand ⁷ Li particles.

For a better understanding of the present invention reference is made tothe following description and tables, the scope of which is described inthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the three-dimensional structure of a preferreddimeric compound of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention includes monomericcompounds having the formula B_(m) I_(n) H_(m-n-1) SH, in which m is aninteger between 5 and 12 and n is a positive integer less than m.Another preferred embodiment includes the dimeric compounds having theformula B_(x) I_(y) H_(x-y-2) S₂, in which x is a positive integerbetween 10 and 24 and y is a positive integer less than or equal to x-2.The preferred compounds of the invention include a periodinatedmonomeric sulfidohydrododecaborane (IBSH) (m=12, n=11) and, morepreferably, a periodinated dimeric sulfidohydrododecaborane (IBSSB)(x=24, y=22). The preferred dimeric compounds of the invention includetwo monomeric boranes linked through a disulfide bridge formed throughthe oxidation of sulfhydryl groups present on the monomers. Thethree-dimensional structure of a preferred dimeric compound of theinvention is illustrated in FIG. 1.

The compounds of the invention should contain at least a naturalabundance of boron-10 isotope. Preferably, at least 95% of the boronatoms in the compound are boron-10 atoms (i.e., 95 atom % boron-10).

The iodine isotopes useful for the invention may be radioactive,preferably iodine-123 (¹²³ I), but may also include ¹³¹ I, or ¹²⁵ I.Alternatively, the iodine isotopes may be radiostable, preferablyiodine-127 (¹²⁷ I). The compounds of the invention may includeradiostable and radioactive nuclides of iodine on a single boranemolecule. The incorporated iodine isotopes permit imaging of theiodinated sulfidohydroboranes by a technique such as CT or SPECT. Thepresent invention also includes methods for preparing iodinatedsulfidohydroboranes as therapeutic agents for use in BNCT.

The invention also includes a method of performing BNCT, which generallyincludes the following steps: administering a boron compound of theinvention to a patient; locating the patient's tumor by scanning thepatient with a device capable of detecting the location and extent ofiodine and thereby, of boron uptake by the tumor; and irradiating thetumor with thermal neutrons to permit neutron capture by the boron andthe subsequent secondary irradiation of the tumor with the fissionproducts of metastable ¹¹ B decay.

Thus, the present invention provides an improved method of treating apatient having a malignant tumor using BNCT. In carrying out the method,a dose of a generally nontoxic or transiently toxic compound isadministered in accordance with the present invention. The compoundcontains at least a natural abundance of, preferably at least about 95%,boron-10 that emits alpha and ⁷ Li particles when bombarded with thermalneutrons. Preferably, the compound is administered to a patientparenterally, e.g., intraperitoneally or intravenously. The compound isadministered in a dosage sufficient to permit the accumulation of theisotope in the tumor to a concentration of at least about 10 ppm,preferably at least about 30 ppm. A neutron source, emitting a beam ofneutrons having an energy distribution effective for neutron capture, isdirected toward the tumor. The patient is positioned so that the tumoris in the irradiation field of the neutron beam for a time sufficient tocause substantial inactivation of tumor cells and/or the cells of thetumor vasculature. One aspect of the present method includes using, asthe compound of the present invention, a non-specific ¹¹⁰ B-carrier, inparticular, an iodinated sulfidohydroborane.

In several proposed BNCT methods, selective accumulation of boroncompounds in specific tissues would be achieved by the use of antibodycarriers for boron compounds. If the antibodies are monoclonal, thesecarriers are selected to specifically target a known antigen on a knowntumor, which would limit their use to particular types of tumors.

One advantage of the borane monomers and dimers of the invention is thatthey are non-specific and therefore are generally useful for a varietyof tumors, particularly brain tumors. The preferential accumulation ofthese boranes in brain tumors is believed to be due to a barrier totheir diffusion from the normal brain tissue (i.e., theblood-brain-barrier). For example, gliomas are often associated withsurrounding edematous brain tissue, likely the result of fluid leakagethrough leaky endothelial walls. Presumably, the leakage of boranes,reversibly bound to plasma proteins (mainly albumin), through theendothelial walls facilitates the diffusion of these compounds to thetumor environment. Meanwhile, these compounds are prevented fromcrossing the relatively impermeable endothelial barrier of non-edematousbrain tissue. It has been observed that the dimeric borane BSSB is morereadily accumulated in brain tumors than the monomeric form, see Slatkinet al., "Boron Uptake in Melanoma, Cerebrum and Blood from Na₂ B₁₂ H₁₁SH and Na₄ B₂₄ H₂₂ S₂ Administered to Mice", Biochem Pharm., 35, 1776(1986) Accordingly, the dimeric iodinated borane IBSSB is likely to bepreferable over the monomeric IBSH. The iodination of boranes, toproduce the compounds of the invention, apparently does not result inserious interference with the tumor affinity of the boranes. This is asignificant advantage over iodinated boron-containing antibodies, theaffinity of which can be compromised by iodination.

As described above, the ¹⁰ B nucleus has a high probability of absorbinga nearby neutrons. The resulting metastable nucleus (*¹¹ B) decays,emitting short range high LET alpha (i.e., ⁴ He) and lithium-7 (⁷ Li)ionizing particles. Since the tissue penetration of these fragments isin the range of less than 10 micrometers, i.e., one-hundredth of onemillimeter, the reaction is significantly destructive only to cells inthe close vicinity of the decaying ¹¹ B nuclei. The 480 keV gamma raysemitted by the decay are locally almost non-destructive to tissues andplay an insignificant role in the therapeutic efficacy of BNCT. If theboron-containing compound is taken up selectively by a tumor, then thetissue ablation will be limited to the tumor site, to selectivelydestroy the cancer cells.

To accumulate the requisite amount of the iodinated borane compounds ofthe present invention in a tumor, generally a systemic dose of betweenabout 10-50 mg ¹⁰ B per kg body weight in a pharmaceutically acceptablecarrier is administered to a patient. The compound is administered inone or more doses, the last dose being given between about 1 hour andabout one week prior to subjecting the patient to the neutron beam. Thequantity of the borane compound used in any particular treatment dependson, among other circumstances, the boron-10 content of the compound andthe toxicity of the compound. The timing of the neutron exposure dependsupon the concentration in blood, which decreases more rapidly with timethan does the tumor concentration. In the event that the tumor-to-bloodratio is less than one, it can be increased by plasmapheresis, aclinically routine procedure. Multiple doses of the iodinated boranecompounds of the present invention can be administered over a period ofa few days prior to radiation treatment. The timing of theadministration of the compound depends on various considerations thatare well known to those skilled in the art of clinical BNCT, includingthe pharmacokinetic behavior of the compound, e.g., the rate ofabsorption of the compound into the tumor and into the tumorvasculature, and the rate of excretion and/or metabolism of the compoundby the patient.

Methods have been proposed to noninvasively localize compounds forneutron-capture therapy, see Slatkin, "A History of Boron NeutronCapture Therapy of Brain Tumors", Brain, 114, 1609 (1991); Hainfeld,"Uranium-loaded Apoferritin with Antibodies Attached: Molecular Designfor Uranium Neutron-Capture Therapy", Proc Natl Acad. Sci. USA, 89,11064 (1992); Martin et al., "Induction of Double-Strand BreaksFollowing Neutron Capture by DNA-Bound Gd-157", Int J Rad Biol Phys ChemMed, 54,205 (1987); Shih et al., "Gadolinium as a Neutron CaptureTherapy Agent", Med. Phys., 19, 733 (1992). Nuclides such as ¹⁵⁷ Gd,^(99m) Tc, ¹¹¹ In, and ⁵⁵ Mn generally form exchangable ionic ratherthan non-exchangable covalent bonds. It is desirable, however, to havean imaging nuclide bound covalently, and therefore stably, in vivo to aneutron-capture agent to assure correspondence between the localizationof the imaging nuclide and the distribution of the capture agent.Because imaging nuclides such as isotopes of iodine (e.g., ¹²³ I and ¹²⁷I) form pharmacologically stable covalent bonds, an iodine-labeled .sup.10 B-carrier would enable the biodistribution of ¹⁰ B to be imagednoninvasively in patients to optimize and enable rapid quantification ofneutron exposure for BNCT.

The calculated weight ratio of iodine to ¹⁰ B in IBSSB, a periodinatedsulfidohydrododecaborane dimer of the invention, is about 11.5. As aresult, the presence of this compound in a tumor, at a therapeuticallyadequate concentration of ¹⁰ B, for example, 30 μg ¹⁰ B per gram tumor,will be associated with 345 μg I per gram tumor. This concentration issuitable for imaging a tumor in the brain and for quantifying the iodinein the image using CT without requiring a conventional contrastenhancement agent. Contrast enhancement in malignant gliomas istypically from about 1 to about 10 Hounsfield units (1 Hounsfield unitcorresponds to 154 μg I/mL tissue, and 1 Hounsfield unit=2 EMI scaleunits, see Gado et al., "An Extravascular Component of ContrastEnhancement in Cranial Computed Tomography", Radiology, 117,589 (1975).It follows that each added Hounsfield scale contrast enhancement unitobserved in a tumor image pixel by CT after 95% ¹⁰ B-enriched IBSSBadministration would correspond to an additional 13.4 μg ¹⁰ B/mL in thetumor zone corresponding to that pixel. Thus, the use of nonradioactive¹⁰ B-IBSSB as a combined radiographic contrast agent and ¹⁰ B-carrierpermits the use of ¹⁰ B concentrations effective for BNCT of malignantgliomas, while simultaneously permitting quantification by conventionalCT.

Alternatively, the use of IBSSB in which the iodine is radioactiveallows visualization and quantification of mere traces of iodine in thetumor by SPECT. Therefore, the mode of imaging used may vary dependingon the isotope of iodine that is used. In practice, it may prove usefulto combine radiostable and radioactive isotopes of iodine on the sameIBSSB molecule. Similarly, a therapeutic composition having a mixture ofthe invented compounds, each possessing one or more stable orradioactive isotopes of iodine, is within the scope of the invention.

For BNCT research and preclinical studies, the microdistribution ofboron delivered by ¹²⁵ I-labeled IBSSB is assessed using ¹²⁵ I-mediatedsilver-grain autoradiography in a contiguous photographic emulsion. Agrain/disintegration efficiency approaching 50% is attainable, providingspatial resolution comparable to that feasible with tritium-basedautoradiography. See Rogers, Techniques of Autoradiography, Third Ed.,Elsevier/North-Holland Biomedical Press, Amsterdam (1979).

The radiation beam clinically useful, for example, for irradiatingintracranial tumors according to the present invention, is a beam ofepithermal neutrons, i.e., neutrons possessing kinetic energies ofbetween 0.4 eV and 10 keV. For example, the epithermal neutron source atthe Medical Research Reactor at Brookhaven National Laboratory, inUpton, N.Y., has been found to produce sufficient quantities of neutronshaving energies effective for BNCT in accordance with the invention. Inparticular, a beam from this source delivering a dose of 8 MW/min. hasbeen found to be effective, as described by U.S. Pat. No. 5,144,026 toGabel, incorporated by reference herein. In general, the beam mustdeliver neutrons which, at the tumor site, have an energy distributionsufficient to permit neutron capture by the boron-10.

To take advantage of the neutron capture phenomenon and the associatedsecondary irradiation, the beam cross-section is preferably larger thanthe diameter of the tumor being irradiated. Given the usual >3cmdiameter of brain tumors, the beam is most preferably on the order ofabout 5-10 cm in diameter. The patient is positioned in front of theneutron beam port so that the tumor is within the neutron irradiationfield. The physical qualities of the beam and the boron distribution inand around the target tissue ensure that the radiation exposure ofhealthy tissue is minimized, while the exposure of tumor tissue ismaximized.

EXAMPLE 1

In one method, compounds of the present invention have been synthesizedby treatment of B₂₄ H₂₂ S₂ ⁴⁻ (either the sodium or potassium form) withan appropriate quantity of iodine and iodine monochloride in suitablesolvents such as methanol/water or a chlorinated hydrocarbon such as1,1,2,2-tetrachloroethane. The synthesis of the periododisulfide isshown in Scheme I. ##STR1##

CS₄ B₁₂ H₁₁ SSB₁₂ H₁₁ (100 mg, 0.114 mmol) (obtained from CalleryChemical Co., Callery, PA) was ion-exchanged to the K⁺ salt (˜57 mg).This was allowed to stir in a round-bottomed flask with I₂ (124 mg, 0.49mmol) in 1,1,2,2-tetrachloroethane (4 mL) at room temperature for 15min. under nitrogen. 1.0 M ICl in dichloromethane (3.6 mL, 3.6 retool)was added and allowed to stir at room temperature for 2 hrs. Thesolution was then heated to reflux for 32 hrs. After cooling, thesolution was filtered. The solid was collected and dssolved in 1N NaOHuntil the dark red-brown color turned yellow or was slightly basic. Thesolution was filtered and to the filtrate was added Me₄ NCl (˜0.10 g in5 mL water) which caused precipitation of the dimer. The product wascollected and purified by recrystallization in 24% molar yield fromBSSB. The beige-colored product was characterized by elemental analyses(Galbraith Laboratories Inc., Knoxville, Tennessee) and infrared (IR)spectroscopy, and shown to be most likely the tetramethylammonium saltof the periododimer (Me₄ N)₄ I₁₁ B₁₂ SSB₁₂ I₁₁ [(Me₄ N)₄ IBSSB]. No BSSBwas identified in the recrystallized product by thin layerchromatography.

The infrared spectrum of IBSSB showed a marked reduction in intensity ofthe strong absorbance at about 2500 cm⁻¹ (corresponding to BHstretching) from that of BSSB. This could be due to a minorcontamination by BSSB or to incomplete iodination. Two absorbancesappeared at 968 and 944 cm⁻¹ in IBSSB which were not present in thespectrum of BSSB. These correlate with BI stretching frequencies in B₁₂I₁₂ ⁻² (940 and 925 cm⁻¹) see Knoth, et. al, "Chemistry of Boranes IXHalogenation of B₁₀ H₁₀ ⁻² and B₁₂ H₁₂ ⁻² ", Inorg Chem., 3,159 (1964).The mercapto monomer can be obtained from the dimer using a reducingagent such as dithiothreitol and the cation can be exchanged to sodiumwith ion exchange chromatography.

EXAMPLE 2

An alternative method for the synthesis of these compounds according tothe present invention, shown in Scheme II, is carried out by iodinationof the hydroborane, B₁₂ H₁₂ ²⁻, followed by sulfurization of theiodohydroborane with N-methyl-benzothiazole-2-thione ("S" in Scheme II),for example, according to the method of Tolpin et al., "Synthesis andChemistry of Mercaptoundecahydro-closo-dodecaborate (2-)", Inorg. Chem.,17, 2867-2873 (1978). The iodo dimer (IBSSB) is synthesized from theresulting iodo monomer (IBSH) by oxidation. ##STR2##

Periodination of B₁₂ H₁₂ ⁻² is carried out under reaction conditionssimilar to those used for BSSB, see, for example, the method describedby Knoth, et al., "Chemistry of Boranes. IX. Halogenation of B₁₀ H₁₀ ⁻²and B₁₂ H₁₂ ⁻² ", Inorg. Chem., 159 (1964).

EXAMPLE 3

K₄ B₁₂ H₁₁ SSB₁₂ H₁₁ →[Me₄ N]₄ B₁₂ H₁₀ ISSB₁₂ H₁₀ I

Iodine (0.20 mol) in methanol (400mL) is added to a solution of B₁₂ H₁₁SSB₁₂ H₁₁ (0.10 mol) in 3:1 methanol:water (200mL). The solution isneutralized with ammonium hydroxide and excess aqueoustetramethylammonium chloride is added. The resulting white precipitateis recrystallized from water. See Knoth et al., "Chemistry of Boranes IXHalogenation of B₁₀ H₁₀ ⁻² and B₁₂ H₁₂ ⁻² ". Inorg. Chem., 3,159 (1964).

EXAMPLE 4

K₄ B₁₂ H₁₁ SSB₁₂ H₁₁ →Cs₄ B₁₂ H₉ I₂ SSB₁₂ H₉ I₂

The same procedure is used for tetraiodination as with the diiodination(Example 3) except that two equivalents of iodine are used instead ofone, e.g., 0.40 tool iodine.

EXAMPLE 5

The tetramethylammonium salt of the periodinated dimer t0 described inExample 1 was ion exchanged to the sodium salt for a toxicity study inmice. Three groups of female mice (BNL Hale-Stoner strain) were given asingle intraperitoneal (i.p.) injection of:

A) 17 μg B per gram of body weight (gbw) as Na₄ BSSB (6 mice),

B) 50 μg B/gbw as Na₄ IBSSB (4 mice), or

C) 0.9% NaCl in water (saline) (7 mice).

Injection volumes were all 0.015 mL/gbw. Five days after the injection,mice were euthanized by deep halothane inhalation anesthesia.Thoracotomy was performed, and about 0.7-0.9 mL blood was removedthrough the right ventricle. Hematological and chemical analyses wereperformed on blood specimens from each mouse.

Hematologic analyses were performed on the blood of mice given BSSB(Group A), IBSSB (Group B) or saline (Group C) in doses described above.The results of these analyses are shown in Table 1. The followingabbreviations are used in Table 1: mouse group and number [MS],leukocytes [WBC] (10³ /μL), erythrocytes [RBC] (10⁶ /μL) , hemoglobin[HGB] (g/dL) , hematocrit [HCT] (%), mean erythrocyte volume [MCV](μm³), mean erythrocyte hemoglobin [MCH] (pg), mean erythrocytehemoglobin concentration [MCHC] (100•σMCH/MCV %), erythrocyte volumedistribution width [RDW] (100•σ/MCV %), platelets [PLT] (10³ /μL), meanplatelet volume [MPV] (μm³), granulocytes [GRN] (10³ /μL), lymphocytes[LYM] (10³ /μL), and midrange-size leukocytes [MID] (10³ /μL).

                                      TABLE 1                                     __________________________________________________________________________    Hematologic analyses of the blood of mice given BSSB                          (Group A), IBSSB (Group B) or saline (Group C)                                MOUSE                                                                              WBC RBC                                                                              HGB HCT                                                                              MCV MCH MCHC                                                                              RDW PLT                                                                              MPV GRN LYM MID                         __________________________________________________________________________    A1   3.4 7.76                                                                             14.8                                                                              42.2                                                                             54.4                                                                              19.1                                                                              35.0                                                                              24.8                                                                              1090                                                                             4.80                                                                              0.10                                                                              3.2 0.10                        A2   5.1 8.81                                                                             7.1 49.2                                                                             55.8                                                                              19.4                                                                              34.8                                                                              24.9                                                                              1382                                                                             4.80                                                                              0.40                                                                              4.2 0.50                        A3   4.0 7.74                                                                             15.5                                                                              44.1                                                                             57.0                                                                              20.0                                                                              35.1                                                                              23.9                                                                              1101                                                                             4.90                                                                              0.30                                                                              3.4 0.30                        A4   4.9 8.81                                                                             17.0                                                                              47.9                                                                             54.4                                                                              19.3                                                                              35.5                                                                              23.7                                                                              1391                                                                             4.80                                                                              0.30                                                                              4.5 0.10                        A5   5.6 8.46                                                                             16.7                                                                              47.7                                                                             56.4                                                                              19.7                                                                              35.0                                                                              24.2                                                                              1316                                                                             4.50                                                                              0.70                                                                              3.9 1.0                         B1   2.6 8.05                                                                             15.6                                                                              44.8                                                                             55.6                                                                              19.4                                                                              34.8                                                                              24.2                                                                              1405                                                                             4.70                                                                              0.20                                                                              2.2 0.20                        B2   4.8 8.28                                                                             15.2                                                                              45.7                                                                             55.2                                                                              18.4                                                                              33.3                                                                              23.6                                                                              1395                                                                             5.10                                                                              0.20                                                                              4.5 0.10                        B3   4.4 7.92                                                                             15.5                                                                              43.4                                                                             54.8                                                                              19.6                                                                              35.7                                                                              23.9                                                                              1536                                                                             4.80                                                                              0.20                                                                              4.0 0.20                        C1   4.7 7.88                                                                             15.6                                                                              44.5                                                                             56.5                                                                              19.8                                                                              35.1                                                                              23.9                                                                              1384                                                                             4.50                                                                              0.30                                                                              4.2 0.20                        C2   4.4 8.72                                                                             17.5                                                                              48.7                                                                             55.9                                                                              20.1                                                                              35.9                                                                              23.8                                                                              1223                                                                             4.80                                                                              0.20                                                                              3.9 0.30                        C3   3.7 8.66                                                                             16.9                                                                              48.2                                                                             55.7                                                                              19.5                                                                              35.1                                                                              24.3                                                                              1334                                                                             5.00                                                                              0.50                                                                              2.6 0.60                        C4   3.3 8.64                                                                             16.7                                                                              48.3                                                                             55.9                                                                              19.3                                                                              34.6                                                                              24.5                                                                              1249                                                                             4.80                                                                              0.30                                                                              2.8 0.20                        C5   4.4 8.85                                                                             17.4                                                                              50.1                                                                             56.6                                                                              19.7                                                                              34.7                                                                              23.9                                                                              1222                                                                             4.80                                                                              0.40                                                                              3.6 0.40                        __________________________________________________________________________

Chemical analyses were performed on blood plasma from mice given BSSB(Group A), IBSSB (Group B) or saline (Group C) in doses described above,the results of which are shown in Table 2. The following abbreviationsare used in Table 2: mouse group and number [MS], glucose [GLU] (mg/dL),blood urea nitrogen [BUN] (mg/dL), blood creatinine [BCR] (mg/dL)aspartate transaminase [AST] (U/L), alanine transaminase [ALT] (U/L),alkaline phosphatase [ALP] (U/L), creatine phosphokinase [CPK] (U/L),total protein [TPR] (g/dL), albumin [ALB] (g/dL), calcium [CA] (mg/dL),chloride [CHL] (meq/L), inorganic phosphorus [P] (mg/dL), potassium [K](meq/L), and sodium [SOD] (meq/L).

                                      TABLE 2                                     __________________________________________________________________________    Chemical analyses of blood plasma from mice given BSSB (Group A),             IBSSB (Group B) or saline (Group C).                                          MOUSE                                                                              GLU BUN                                                                              BCR                                                                              AST                                                                              ALT                                                                              ALP                                                                              CPK                                                                              TPR                                                                              ALB                                                                              CA CHL                                                                              P  K  SOD                              __________________________________________________________________________    A1   311 16 0.4                                                                              46 28 128                                                                              50 5.7                                                                              3.4                                                                              11.7                                                                             119                                                                              11.7                                                                             10.0                                                                             156                              A2   217 26 0.4                                                                              41 21 115                                                                              37 5.3                                                                              3.3                                                                              10.9                                                                             118                                                                              8.8                                                                              6.9                                                                              154                              A3   315 23 0.4                                                                              56 36 130                                                                              64 6.0                                                                              3.6                                                                              11.1                                                                             117                                                                              9.6                                                                              9.2                                                                              154                              A4   268 23 0.5                                                                              70 34 103                                                                              141                                                                              5.3                                                                              3.2                                                                              11.3                                                                             119                                                                              10.9                                                                             9.9                                                                              153                              A5   249 23 0.6                                                                              80 28 132                                                                              215                                                                              6.0                                                                              3.6                                                                              11.6                                                                             121                                                                              9.0                                                                              6.5                                                                              161                              A6   269 30 0.5                                                                              40 19 143                                                                              58 5.8                                                                              3.4                                                                              11.0                                                                             120                                                                              9.4                                                                              7.2                                                                              158                              B1   332 15 0.4                                                                              80 33 108                                                                              89 5.5                                                                              3.0                                                                              11.2                                                                             119                                                                              10.1                                                                             7.9                                                                              155                              B2   206 27 0.4                                                                              77 47 123                                                                              148                                                                              5.8                                                                              3.3                                                                              11.9                                                                             119                                                                              10.8                                                                             7.8                                                                              156                              B3   252 26 0.4                                                                              89 66 123                                                                              75 5.8                                                                              3.2                                                                              11.3                                                                             117                                                                              9.8                                                                              6.0                                                                              155                              B4*  278 28 0.5                                                                              44 22 110                                                                              35 5.9                                                                              3.3                                                                              11.5                                                                             116                                                                              9.9                                                                              7.1                                                                              154                              C1   307 24 0.5                                                                              63 26 149                                                                              134                                                                              5.6                                                                              3.0                                                                              11.6                                                                             121                                                                              11.7                                                                             11.3                                                                             153                              C2   298 17 0.4                                                                              58 26 156                                                                              113                                                                              5.3                                                                              3.2                                                                              11.1                                                                             121                                                                              11.0                                                                             11.0                                                                             154                              C3   286 31 0.5                                                                              49 26 120                                                                              36 5.5                                                                              3.2                                                                              11.3                                                                             115                                                                              8.8                                                                              6.3                                                                              154                              C4   295 28 0.5                                                                              41 28 112                                                                              35 5.5                                                                              3.3                                                                              11.1                                                                             119                                                                              9.0                                                                              6.6                                                                              155                              C5** 234 32 0.5                                                                              112                                                                              54 126                                                                              245                                                                              5.9                                                                              3.5                                                                              11.4                                                                             123                                                                              9.7                                                                              9.5                                                                              158                              C6   226 33 0.5                                                                              50 19 136                                                                              99 6.1                                                                              3.6                                                                              11.6                                                                             121                                                                              10.1                                                                             7.9                                                                              159                              C7   410 29 0.6                                                                              30 10 164                                                                              58 5.8                                                                              3.3                                                                              12.0                                                                             121                                                                              11.2                                                                             10.5                                                                             157                              __________________________________________________________________________     *Mouse not given entire dose of IBSSB; excluded from analyses.                **Mouse appears to have had liver dysfunction by its abnormal liver enzym     values; excluded from analyses.                                          

As shown in Tables 1 and 2, a single i.p. injection of IBSSB at a dosecorresponding to 50 μg B/gbw appears to have been only minimally moretoxic than a similar injection of three-fold less BSSB, as judged byhematological and chemical tests of the blood 5 days later.Inadvertently, the BSSB group (Group A) received 17 μg ¹⁰ /gbw, onlyabout one-third of the intended boron dose, 50 μg/gbw. Nevertheless, itis useful to compare the subacute toxicity of a relatively high dose ofa substance, IBSSB, which had never before been tested in an animal,with that of its parent compound, BSSB, which is known to be non-lethalat about 35 μg B/gbw, i.e., twice the lower of the two boron doses used.See Slatkin et al., "Boron Uptake in Melanoma, Cerebrum and Blood fromNa₂ B₁₂ H₁₁ SH and Na₄ B₂₄ H₂₂ S₂ Administered to Mice", Biochem Pharm,35, 1771 (1986)

The fourteen chemical and thirteen hematological tests of blood frommice given either BSSB (Group A) or IBSSB (Group B) showed nosignificant differences between the two groups using the WilcoxonTwo-Sample test, See Miura et al., "Biodistribution and Toxicity of2,4-Divinyl-nido-carboranyldeuterophorphyrin IX in Mice", Biochem Pharm,43,467 (1992) It therefore appears that the periodinated borane is notmore toxic than the non-iodinated borane.

Alanine transaminase [ALT] levels in the mice given IBSSB (Group B) weresignificantly higher than those in mice given physiological saline(Group C). No other differences were noted between mice in groups B andC. We surmise from this preliminary in vivo experiment that IBSSB may beslightly hepatotoxic to mice, but there is no evidence of significanttoxicity at blood concentrations believed to be useful for BNCT.

Thus while we have described what are presently the preferredembodiments of the present invention, other and further changes andmodifications could be made without departing from the scope of theinvention, and it is intended by the inventors to claim all such changesand modifications.

We claim:
 1. An iodinated sulfidohydroborane having the chemicalformula:B_(m) I_(n) H_(m-n-1) SH wherein B_(m) includes at least about20 atom % boron-10 isotope, and wherein m is an integer between 5 and12, and n is a positive integer less than m.
 2. The iodinatedsulfidohydroborane of claim 1, wherein m=12 and n=11.
 3. The iodinatedsulfidohydroborane of claim 1, wherein m=12 and n=1.
 4. The iodinatedsulfidohydroborane of claim 1, wherein m=12 and n=2.
 5. The iodinatedsulfidohydroborane of claim 1, wherein I is a radiostable isotope ofiodine.
 6. The iodinated sulfidohydroborane of claim 1, wherein B_(m)includes at least about 95 atom% boron-10 isotope.